well g40s use the same size master cylinder as the rest of mk3 polos do. You can fit a ~20mm one from a mk2 golf 16v, passat etc... it'll reduce pedal travel but wont make your brakes better as such, in fact you'll have to press harder to get the same effort.
what brakes do you currently have on it? As a minimum you want the 239mm vented discs like g40s do and then some decent pads.
